What is the Appropriate Control for the Tilt Illusion?

Recently it has been suggested that when tilt illusions are measured by parallel matching to the test arm of an acute angle, the usual control condition involving a match to the test arm in the absence of the inducing arm is inappropriate. According to Hotopf et al this is so because the pretest contains a second, depth-based illusory effect which is not contained in the test conditions. Whereas Hotopf et al gave indirect evidence for this claim, direct evidence is presented here for their assertion. The results suggest that there is no single stimulus configuration which could serve as a pretest control for all tilt illusion stimuli. Rather, the condition in which the test and inducing lines intersect at 90° probably is the appropriate control for all other inducing-test-line angle displays.